Interest has increased in transmitting acoustic signals to and from locations in an oil well environment. The basic operating principal in acoustic signal transmission in a tubular media is to impart propagating stress-strain waves into a pipe or tubing string which travel within the pipe to a distant location where transducers detect the signal which is then interpreted by the receiving equipment. In this way, data and signals can be transmitted via mechanical tubular transmission channels such as pipe or tubing.
There are practical problems in the transmission of acoustic signals. When tubing, drill pipe or casing is used as an acoustic transmission channel, significant signal distortion often occurs, due to reflective interfaces in the channel such as tool joints, collars or other upsets. Additionally, there can be significant attenuation and interference associated with the fluid system contained in the wellbore and echoes of the acoustic signals themselves within the wellbore. The receiver is subjected to noise from the operation of surface equipment and other external disturbance sources, as well as from fluid flow and other noise present in the acoustic channel. These factors significantly reduce the conditions under which acoustic data transmission may be effectively utilized. Acoustic data transmission may be limited by the distance of the transmission, the number and type of upsets in a drill string.
Efforts to effectively transmit data acoustically have often centered on careful control of the frequency and bandwidth of the transmission, the timing of the transmission and the duration of the transmission. U.S. Pat. No. 3,252,225 issued to Hixon and U.S. Pat. No. 4,314,365 issued to Petersen teach selection of transmission wave length based upon pipe characteristics such as the length of pipe sections and the overall length of the drill string. U.S. Pat. No. 4,390,975 issued to Shawhan suggests delaying successive acoustic data transmissions to allow reflections of earlier transmissions to dissipate. Similarly, U.S. Pat. No. 5,050,132 issued to Duckworth discloses transmissions of acoustic data signals only during pre-selected short time intervals to avoid data distortion. U.S. Pat. No. 5,124,953 issued to Grosso discloses selecting a passband frequency for acoustic data transmission that best correlates a measured and a modeled spectral density of the acoustic transmission. U.S. Pat. No. 5,148,408 issued to Matthews similarly suggests the testing and finding of an optimum frequency for acoustic data transmission which results in the most efficient reception of the acoustic data under the circumstances then present in the well. The Matthews patent suggested period testing of data transmission through the drill string during drilling operations, finding an optimum frequency for transmission based upon drill string conditions at the time of testing, and changing the acoustic data transmission frequency as needed. U.S. Pat. No. 4,562,559 issued to Sharp et al, proposes a phase-shifted transmission wave having a broader frequency spectrum to bridge gaps in the passband frequencies. U.S. Pat. No. 5,128,901 issued to Drumheller proposes transmission of acoustic data conditioned to counteract interference caused by the drill string so as to enhance data transmission.
In some systems it is possible to model a communication channel before the system is placed in service and design an acoustic transmitter to compensate for the channel distortion. However, in an oil well, complexities of the acoustic transmission environment change constantly, defying any attempt to design a workable static acoustic transmitter. A receiver embodying a single sensor must accomplish all of the noise reduction in the electronics.
From the foregoing, it is apparent that a need exists for improved methods of acoustic data transmission and, in particular, a need exists for improved methods of acoustic data transmission in oil well environments. Furthermore, it would be most desirable to provide such methods for reinforcement of the basic control signal and attenuation of unwanted interference. If the noise is high, it can swamp the available signal. Thus, it is particularly desirable to have a method of reducing the noise prior to the digitization of the signal so as to increase the range and reliability of acoustic data transmission.